The present invention relates to a chisel for a crust breaking facility for breaking open the crust of solid electrolyte on an electrolytic cell, in particular on a cell for producing aluminum.
In the manufacture of aluminum from aluminum oxide, the latter is dissolved in a fluoride melt made up for the greater part of cryolite. The aluminum which separates out at the cathode collects under the fluoride melt on the carbon floor of the cell; the surface of this liquid aluminum acts as the cathode. Dipping into the melt from above are anodes which, in the conventional reduction process, are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is produced at the carbon anodes; this oxygen combines with the carbon in the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of approximately 940.degree.-970.degree. C.
The concentration of aluminum oxide decreases in the course of the process. At an Al.sub.2 O.sub.3 concentration of 1-2 wt. % the so-called anode effect occurs producing an increase in voltage from e.g. 4-4.5 V to 30 V and more. Then at the latest the crust must be broken open and the concentration of aluminum oxide increased by adding more alumina to the cell.
Under normal operating conditions the cell is fed with aluminum oxide regularly, even when no anode effect occurs. Also whenever the anode effect occurs the crust must be broken open and the alumina concentration increased by the addition of more aluminum oxide; this is called servicing the cell.
For many years now servicing the cell includes breaking open the crust of solidified melt between the anodes and the side ledge of the cell and then adding fresh aluminum oxide. This procedure, which is still widely practiced today, is being criticized increasingly because of the pollution of the air in the pot room and the air outside. In recent years therefore it has become increasingly necessary and obligatory to hood over or encapsulate the reduction cells and to treat the exhaust gases. It is however not possible to capture completely all the exhaust gases by hooding the cells if the cells are serviced in the classical manner between the anodes and the sides of the cells.
More recently therefore aluminum producers have been going over to servicing the cells at the longitudinal axis of the cell. After breaking open the crust, the alumina is fed to the cell either locally and continuously according to the point feeder principle or discontinuously along the whole of the central axis of the cell. In both cases a storage bunker for alumina is provided above the cell. The same applies for the transverse cell feeding proposed recently by the applicant in U.S. Pat. No. 4,172,018.
The breaking open of the solidified electrolyte is carried out with conventional, well known devices provided with chisels which are rectangular or round in cross section.
The lower part of the chisel which comes into direct contact with the crust on breaking it open is shaped as follows:
flat end face lying perpendicular to the sidewalls of the chisel (Swiss Pat. No. 520,778). This shape of chisel can be regarded as the normal shape, PA1 round chisel with a conical point (German Pat. No. 2,135,485), PA1 flat conical end to a round chisel (U.S. Pat. No. 3,371,026).
The disadvantage of flat ended chisels is that the relatively hard and thick electrolyte crust has to be pushed down at the same time over the whole cross section of the tool. With chisels which have tapered ends the vertical force to be applied is indeed smaller. However, because of the wedge effect of the inclined sides, there are still significant forces acting sideways to be overcome. This increases considerably the energry required, and the investment costs.